1. Field of the Invention
The present disclosure relates to a drive circuit for driving a light emitting device and a vehicle lamp using the light emitting device drive circuit.
2. Background Art
In recent years, semiconductor light emitting devices such as LEDs (light emitting diodes) and LDs (laser diodes) have been in use as light sources for vehicle lamps. Vehicle lamps include drive circuits for supplying stable current to semiconductor light emitting devices in order to drive the semiconductor light emitting devices.
While there may be a case where the temperature of a vehicle lamp is increased by natural environmental factors such as outside air temperature and sunlight, as well as heat radiated from the engine in the engine bay, in a semiconductor light emitting device, in the event that the temperature thereof exceeds its maximum rated temperature, deterioration in luminance progresses rapidly. In other words, the life of the semiconductor light emitting device is decreased. Accordingly, in a drive circuit described in Patent Document No. 1, the temperature of a vehicle lamp or preferably, a temperature in a location lying in the vicinity of a semiconductor light emitting device, is detected and a current that is supplied to the semiconductor light emitting device is decreased based on the temperature so detected. By doing so, a rise in temperature of the semiconductor light emitting device being thereby suppressed. (see, e.g., Japanese Unexamined Patent Document: JP-A-2004-276738)
However, there are some models in which it becomes difficult to dispose the drive circuit in the vicinity of the semiconductor light emitting device. In this case, wiring is necessary to connect a temperature detecting device disposed in the vicinity of the semiconductor light emitting device to the drive circuit and space to dispose the temperature detecting device and a fixing member for the relevant device are also necessary, whereby the production cost of the relevant vehicle lamp is increased. In order to avoid such an increase in the production cost, the temperature of the semiconductor light emitting device could be estimated by detecting an output voltage of the drive circuit as a forward voltage of the semiconductor light emitting device, however, in detection of an absolute value of the voltage, the detection accuracy is decreased due to variation in forward voltage of individual semiconductor light emitting devices. On the other hand, in detection of a relative value of the voltage, although the decrease in detection accuracy attributed to the variation in forward voltage in the individual semiconductor devices can be suppressed to a lower level, the forward voltage of the semiconductor device needs to be stored in advance. As a result, memory and a peripheral circuit become necessary, and this raises again the issue of increase in the production cost of the vehicle lamp.